A drawing technique for drawing an optical fiber from an optical fiber preform having a large diameter (e.g., 70 mm) has been coming into common use due to demands for cutting down the cost of optical fiber, and the like. In the case where an optical fiber preform having a large thickness is to be drawn, the space surrounding its neck-down part becomes greater, whereby the gas flowing through this space exhibits an uneven temperature distribution. As a consequence, turbulence occurs in the gas flow in the space near the neck-down part, whereby the optical fiber diameter fluctuates greatly. For restraining the optical fiber diameter from fluctuating, there is a case where an He gas having a high thermal conductivity is used as the atmosphere gas within the drawing furnace.
Also known is a technique in which the drawing furnace is provided with a muffle tube extension (also called as a lower chimney) in order to isolate the optical fiber immediately after drawing from the outside air.